In lithography for manufacturing semiconductor devices, the patterns (or structures) on a mask (also called a reticle) are projected by wafer exposure equipment onto a wafer that has a photosensitive layer, called the photoresist. When a microscope is used for mask inspection, the mask pattern is projected onto a photosensitive spatially resolved detector, such as, for example, a charged coupled device (CCD) sensor. In doing so, the pattern is enlarged by a factor of, e.g., 150 to 450, in order to more accurately detect any defects present in the patterns. When projecting the pattern onto the wafer, the pattern is imaged and reduced in size. For example, in modern scanners, the pattern is reduced by a factor of four.
Because defects that also occur during wafer exposure are primarily of interest during mask inspection, the aerial images produced in the resist and on the detector should be as identical as possible, except for the different magnification scale. In order to achieve equivalent image production, the wavelength of projection light, illumination settings, and the numerical aperture (NA) used on the object side, i.e., mask-side, are adjusted to be the same as, or as similar as possible to, the scanner used.
Mask inspection microscopes can operate in transmission or reflection. The mask pattern image is created after the projection light is transmitted through the mask (in the case of a transmission mask) or after the projection light is reflected from the mask surface (in the case of a reflective mask).
The projection light is optimized in scanners for wafer exposure in accordance with the patterns of the masks to be imaged in each case. Various lighting settings are used to define the lighting intensity distribution on a pupil plane of the light beam path of the mask inspection microscope. It is common to use various lighting settings having different degrees of coherence, as well as off-axis lighting settings such as annular illumination and dipole or quadrupole illumination. The depth of field or resolution can be increased by lighting settings that produce off-axis oblique lighting.
With microscopes that are used to examine masks, optics having a smaller field of view and a smaller illumination area are used compared to those of wafer exposure equipment.
In a method for optimizing masks, local density variations are introduced into the mask that function as scattering centers. The transmission and reflectivity of the mask are modified by the local density variations. This is used, for example, to optimize the critical dimension (CD) of a mask pattern, as well as the placement error (registration), i.e., the position of the mask patterns. The critical dimension refers to the dimension of a critical feature on the wafer, such as minimum line width of single lines, minimum line width of densely spaced lines, or the minimum size of contact holes. The projected light is attenuated through local density variations, hereinafter also called pixels. The intensity of an aerial image of the mask can be modified locally by the attenuation, e.g., by weakening or reduction of the transmission. This enables optimization of the critical dimension.
In the case of transmissive masks, the scattering centers are introduced into the mask by pulsed femtosecond lasers. This is known, for example, from U.S. published patent application 20070065729. The scattering centers can be introduced into the substrate of the mask with this method.
The introduction of scattering centers into extreme ultra-violet (EUV) masks by using electron beams is known from German patent application DE 10 2011 080 100 A1. In this method, scattering centers are introduced into the reflecting multilayer of the mask in order to reduce reflectivity.